1. Field of the Invention
The present invention relates to a semiconductor waveguide photodetector with high detection sensitivity, which acts also as a filter applicable to the wavelength division multiple communication.
2. Description of the Prior Art
Recently, the optical fiber communication technology has been progressing as one of the technologies for large capacity and high speed transmission. Particularly, the semiconductor photodetector, amongst various optical devices supporting optical communication technology, are of great interest, because it is one of the key devices for implementing long distance communication, which receives directly the transmitted signal and converts the light signal to electrical signal.
There are some variations in the semiconductor photodetector. Particularly, the semiconductor waveguide photodetector is characterized in that it is packaged in a package similar to a semiconductor laser and it is integrated easily with other waveguide optical devices or passive waveguides.
Generally, the light receiving region is designed as a multimode waveguide for improving the receiving efficiency of light signal. Such a multimode waveguide for the semiconductor waveguide photodetector is reported, for example, in IEEE Journal of Quantum Electronics Vol. 28 No. 12, 1992, p2728 to p2735, by K. Kato.
If the only purpose is to receive optical signal, a single mode waveguide is not necessary. However, it is desirable that the waveguide photodetector behaves also as a filter as well as a detector. In case of the wavelength division multiple communication, a semiconductor photodetector is provided with the filtering function which passes the wavelength longer than the sensitive wavelength of the photodetector and absorbs the wavelength within the sensitive band. The above-mentioned function requires a single mode waveguide which propagates single mode light.
As described above, it is preferable that the light receiving region be designed as a multimode waveguide in view of the receiving efficiency of light signal. On the other hand, a single mode waveguide is required in order to obtain the filtering function for the wavelength division multiple communication. These two conditions are contradictory.
Therefore, an object of the present invention is to provide a semiconductor photodetector with higher light receiving efficiency, which has also a function of optical filter.
In accordance with the present invention, there is provided a semiconductor photodetector which outputs single mode light, which comprises a light waveguide structure including a multi mode waveguide region.
The multi mode waveguide region may be a 1xc3x971 multi mode interference light waveguide.
The semiconductor waveguide photodetector may comprise a multi mode waveguide region; and a couple of single mode waveguide regions, each of which is connected to an each end of the multi mode waveguide region.
The width of the multi mode waveguide region is made wider than the width of the single mode waveguide regions.
The above-mentioned structure behaves as a multi mode waveguide light receiving region for the received light and behaves as a single mode waveguide optical filter for the transmitted light.
As described above, the semiconductor waveguide photodetector of the present invention is highly sensitive, because the main light receiving region is a multi mode waveguide. Further, the multi mode waveguide is the 1xc3x971 multi mode interference (MMI) waveguide. Therefore, the semiconductor waveguide photodetector functions also as a filter suitable for the waveguide division multiple communication.